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AMAE Int. J. on Manufacturing and Material Science, Vol. 02, No. 02, May 2012 

A Comparative Study on Direct and Pulsed Current 
Gas Tungsten Arc Welding of Alloy 617 

E. Farahani 1 , M. Shamanian 2 , F. Ashrafizadeh 2 
1 Pars Oil & Gas Company (POGC)/Inspection Engineering Department, Tehran, Iran 

2 Isfahan university of technology/Department of materials engineering, Isfahan, Iran 

Abstract — the aim of this article is to evaluate the mechanical 
and microstructure properties of Inconel 617 weldments 
produced by direct current electrode negative (DCEN) gas 
tungsten arc welding (GTAW) and pulse current GTAW. In 
this regard, the micro structural examinations, impact test 
and hardness test were performed. The results indicated that 
the joints produced by direct mode GTAW exhibit poor 
mechanical properties due to presence of coarse grains and 
dendrites. Grain refining in pulse current GTAW is reason of 
higher toughness and impact energy than DCEN GTAW. 
Further investigations showed that the epitaxial growth is 
existed in both modes that can strongly affect the mechanical 
behavior of the joints in heat affected zone (HAZ). 

Index Terms- Alloy 617, Welding, Pulsed Current, 
Microstructure, Grain Refining. 

I. Introduction 

Super alloys are divided into three groups including iron, 
nickel and cobalt alloys. Inconel is a registered trademark of 
Special Metals Corporation that refers to a family of austenitic 
nickel-chromium based super alloys. Inconels retain their 
mechanical properties at high temperature applications where 
many kinds of steels are susceptible to creep as a result of 
thermally-activated deformation [1-4]. 

Alloy 617 (UNS N00617- ASTM B 166), a solid solution 
nickel-based alloy, has a face-centered-cubic (FCC) crystal 
structure, widely used in the high temperature applications 
because of its excellent high temperature corrosion resistance, 
superior mechanical properties, good thermal stability and 
superior creep resistance [4-6]. 

The microstructure and phase stability of Inconel 617 
alloy were investigated by researchers [7] . They showed that 
M, 3 C 6 carbides can be formed after high temperature 
exposures (in the range of 649 UC -1093UC). Presence of 1 
wt% aluminum also strengthens the matrix by forming Ni Al 
inter metallic compound which slightly improves the 
mechanical properties at 650UC -760UC. However, the major 
role of aluminum and chromium additions is to improve the 
oxidation and carburization resistance at high temperatures 
[8]. The corrosion behavior [9, 10] and high temperature 
properties [8, 11-14] of Inconel 617 have been previously 
investigated in the literature. 

It should be mentioned that welding processes are 
essential for the development of virtually manufactured 
Inconel products. However, the papers which deal with the 
investigation of Inconel 617 weldments are a few. However, 

©2012 AMAE 

DOI: 01 JJMMS.02.02.41 


the microstructure of dissimilar Inconel 617/ 310 stainless steel 
produced by gas tungsten arc welding . (GTAW) has 
beeninvestigated [4] 

Pulsed current GTAW (PCGTAW), developed in the 1950s, 
is a variation of constant current gas tungsten arc-welding 
(CCGTAW) which involves cycling of the welding current 
from a high level to a low level at a selected regular frequency. 
The high level of the pulsed current is selected to give 
adequate penetration and bead contour, while the low level 
of the background current is set at a level sufficient to maintain 
a stable arc. This permits arc energy to be used efficiently to 
fuse a spot of controlled dimensions in a short time. It 
decreases the wastage of heat through the conduction into 
the adjacent parent material [15, 16]. In contrast to CCGTAW, 
during PCGTA, the heat energy required to melt the base 
material is supplied only during peak current pulses (for brief 
intervals of time). It allows the heat to dissipate into the base 
material leading to a narrower heat affected zone (HAZ). The 
PCGTAW has many specific advantages compared to 
CCGTAW, such as enhanced arc stability, increased weld 
depth to width ratio, refined grain size, reduced porosity, low 
distortion, reduction in the HAZ and better control of heat 
input. In general, the PCGTAW process is suitable for joining 
thin and medium thickness materials, e.g. stainless steel 
sheets, and for applications where metallurgical control of 
the weld metal is critical [17]. 

PCGTAW of super alloys is scanty in the reported 
literatures, but some researchers have evaluated the effect of 
pulsed current parameters on corrosion and metallurgical 
properties of super-duplex stainless steel welds [15]. In 
addition, PCGTAW of Ti-6A1-4V titanium alloy, AA 6061 
aluminum alloy and 304L austenitic stainless steel have been 
also reported in the previous papers [18-20]. However, 
PCGTAW of Inconel 617 has not been reported in the 
literature. The aim of this study is to investigate the micro 
structural and mechanical properties of Inconel 617 welds 
produced by GTAW and PCGTAW using Inconel 617 filler 


A. Materials 

Inconel 617 alloys were cut and machined in the form of 
11mm x 110mm x 150mm plates. The solution annealing 
treatment was performed at 1175UC for 1 h and then the 
samples were cooled in turbulent air. The Inconel welds were 


AMAE Int. J. on Manufacturing and Material Science, Vol. 02, No. 02, May 2012 

fabricated using CCGTAW and PCGTAW. In this regard, 
Inconel 617 (UNS N06617, AWS No. ERNiCr22Col2Mo9) filler 
with diameters of 2.6 mm and 1 .4 mm were used in CCGTAW 
and PCGTAW, respectively. The compositions of base metal 
and filler metal are given in Table 1 . 

B. Welding Setup 

The multi-pass fusion welds were carried out on 1 1mm 
thick plate using the manual GTAW process with Magic Wave 
2600- Fornius transformer. The weld joint profile according 
to DIN EN 29692 standard is shown in Fig. 1 . The weld is butt 
type with a v-grooved profile. All butt joints were machined 
into 37. 5U for good diffusion and were completed in three 
passes with shielded argon gas (99.99% purity and 10-15 
lit.min-1 flow rates). The welds were completed in four passes 
with air cooling to at least 200UC between passes. Inter pass 
temperature was measured by a thermocouple attached to 
the welded plate. The pulsed current mode for root pass uses 
the optimized frequency 6 Hz with percent peek- time control 
setting of %70 and the other passes use optimized frequency 
8 Hz with peek time of %80. The mean value of current in 
PCGTAWis calculated with equation (1). Equation (1) is: 


Im = 

(IpTp + IbTb) 


(Tp + Tb) 

Where Im is mean current, Ip and tp are peek current and 
time respectively, lb and tb are background current and time 
respectively. In the pulsed-current mode, the welding current 
rapidly alternates between two levels. The higher current 
state is known as the pulsed current, while the lower current 
level is called the background current. During the period of 
pulsed current, the weld area is heated and fusion occurs. 
Upon dropping to the background current, the weld area is 
allowed to be cooled and solidified. Mean current with this 
formula can use for calculating the heat input in pulsed current 
mode. The welding parameters in all 3 passes of GTAW and 
PCGTAW are shown in Table 2. The efficiency for calculating 
the heat input is 70% in both modes. 

C. Characterizations and Testing 

Two ends of weld sample were discarded; remained part 
was prepared for micro- structural examinations, hardness 
measurements and notch-toughness test as shown in Fig. 2. 
These test specimens were prepared according to ASME Sec. 
K-QW462 standard. In addition, the Charpy V-notch test 
specimens were prepared according to ASTM-A370 sub- size 
(as ASME Sec. IX referred) with dimensions of 10mm x7.5mm 

Table I. Composition (%Wt) of Filler Metal and Base Metal 


AllDy *517 

Fill J -r T-i-i -=t.^ 1 



Hjiljin-i- e-ii 






: z 















. 6 






:« . : 







©2012 AMAE 


1.5 ID [11 

1 mill 

Fig. 1 . Weld Joint Profile 
Table II. Welding Parameters 

12 mm 

Pass Current VoltaE 
Nd. (A) (V)~ 



- input 
















No 2 





No 3 







19 2 







10 B 

17 3 




No 2 





No 3 


16 9 



The Charpy V-notch impact tests were performed on weld at 
room temperature by using Amsler impact test. The samples 
were machined perpendicular to the weld direction with notch 
on the center of weld metal. The impact tests were performed 
on three samples to increase the results of the degree of 

TJjttrrinptE t**ct 

4"hirpy V notrh 





Fig. 2. Preparing the Test Samples 

Standard metallographic procedures were used to prepare 
cross sections of the base metals and joints for micro structural 
characterizations. Sections were etched using aqua regia 
"Marble etchant solution". Vickers hardness measurements 
were performed with dwell time of 5 seconds on face of weld 
metal on three different samples. 

A. Macrostructure 

The welded samples were polished and examined using 


AMAE Int. J. on Manufacturing and Material Science, Vol. 02, No. 02, May 2012 

optical microscope equipped with camera. The photographs 
are shown in Fig. 3. The PCGTAW sample exhibit a narrower 
fusion zone and smaller HAZ as compared to CCGTAW one. 
This is due to the lower heat input and consequently smaller 
fusion zone of the pulsed current mode (as shown in Table 2). 

A. Annealing Twins 

Inconel 617 plates were annealed at 1 175 UC for 1 h and 
cooled at turbulent air in order to dissolve the precipitates. 
Therefore, annealing twins clearly existed in the base metal 
microstructures of two samples. The boundaries of twins are 
ordered and have a coherent interface. As twin's boundaries 
can prevent the dislocations movement during the 
deformation, they will have good effect on the strength of 
cubic materials [21]. Fig. 4 shows the microstructure of weld 
sample and its annealing twins. 

The investigation of heat affected zone shows that with 
moving from base metal toward the weld interface, the grain 
growth is occurred. It can be due to the increase of heat input 
and temperature during welding. As y phase has low heat 
transfer coefficient and high heat capacity, the cooling rate 
will be low and consequently the grain growth occurs. The 
Inconel alloy 617 was supplied in the form of solution treated 
and water- quenched plate. The typical metallographic feature 
of alloy consist of an austenitic matrix with some intergranular 
and transgranular obvious precipitates (Fig. 4). 

Fig.3. Welded Samples of (a) CCGTAW, (b) PCGTAW 

Fig.4. Annealing Twins and Carbides in Base Metal 

The EDS results confirmed that these particles are Cr-rich 
(M, 3 C 6 ), Mo-rich (M 6 C) carbides or a combination of both 

C. Epitaxial Growth and Competitive Growth 

©2012 AMAE 

The epitaxial growth in GTAW and PCGTAW process slightly 
occurs at the weld interface. The epitaxial growth occurs in 
the weld/base metal systems having the same crystal 
structures (here FCC). Far from the fusion line, competitive 
growth is attributed to the growth of grains at different 
directions. Easy growth direction is <100> in FCC materials, 
however, the growth can occur at different directions due to 
presence of different phases [22- 24]. Where metal base is 
ferritic and weld metal is austenitic, the normal epitaxial growth 
may occur parallel to the fusion boundary. This kind of micro 
structural growth was also reported by the others researchers 

In fusion welding, the existing base-metal grains at the 
fusion line act as the substrate for nucleation. Due to complete 
wet ability (9 = 0) between liquid metal and fusion line, the 
nucleation from liquid metal occurs easily. Such a growth 
initiation process is called epitaxial growth or epitaxial 
nucleation [22]. The structure of grains near the fusion line 
and during the solidification is different. Grains tend to grow 
perpendicular to fusion pool boundary due to the more 
thermal gradient in this direction. But the sub-grains tend to 
grow in easy-growth direction. 

Fig. 5. Epitaxial and competitive growth 

Therefore, during the solidification, grains will grow faster. 
This phenomenon is called competitive growth as shown in 
Fig. 5 [22, 26]. Two types of grain boundaries may present 
near the weld metal of dissimilar welds: type I (in a direction 
roughly perpendicular to the fusion boundary caused by 
epitaxial growth) and type II (in a direction roughly parallel to 
the fusion boundary). Type I boundary is usually observed 
in the similar welds (similar bases and filler metals), while 
type II boundary is a result of allotropic transformation in the 
base metal that occurs during cooling of weld in dissimilar 
welds (BCC/FCC). However, in the present study, all the 
micrographs revealed only type I boundaries. It should be 
noted that although dissimilar welds are produced, there is 
no allotropic transformation during the cooling of two base 
metals [23, 24]. 

D. Grain Refining in PCGTAW 

The optical micrographs from microstructure of welded 
samples are shown in Fig. 6. Many large columnar grains 
with dendritic structure existed in CCGTAW specimen. Coarse 
grains can be observed at HAZ of CCGTAW sample (see Fig. 
4). The dendrite spacing of weld metal in the constant current 
is wider but the pulsed current process reveals narrower 


AMAE Int. J. on Manufacturing and Material Science, Vol. 02, No. 02, May 2012 


Areas containing fine equiaxed grains were observed, 
generally located in center of fusion zone. HAZ of the weld 
was characterized by carbide dissolution and grain growth, 
n the case of PCGTAW sample, the grain refinement has 
occurred at fusion zone which can result in better mechanical 
properties. The Charpy V-notch results (impact test) indicated 
that absorbed energy during the fracture of PCGTAW sample 
lis greater than that for CCGTAW sample (as shown in Table 
3). The grain refining in the PCGTAW process can be 
attributed to lower heat input causing the faster cooling rate 
which delays the grain growth. The other reason is the effect 
of pulsed current on dendrites. As shown in Fig. 7, the 
dendritic structure is observed in CCGTAW specimen. It was 
found that dendritic structures are associated with greater 
degree of segregation and are more susceptible to cracking 
[24]. The room temperature strength of PCGTAW sample with 
fine grains is greater - than that of GTAW one with coarse 
grains, as there are no significant chemical difference between 
two samples [27]. In general, the formation of equiaxed grain 

Fig.6. Weld metal microstructure of (a) CCGTAW, (b) PCGTAW 

structure in CCGTAW weld is known to be difficult because 
of the remelting of heterogeneous nuclei or growth centers 
ahead of the solid-liquid interface. This is due to the high 

©2012 AMAE 

temperatures in the liquid, thus making survival nuclei 
difficult. The evolution of microstructure in weld fusion zone 
is also influenced in many ways by current pulsing. Principally, 
the cyclic variations of energy input into the weld pool cause 
thermal fluctuations, one consequence of which is the 
periodic interruption in the solidification process. As the 
pulsed peak current decays the solid-liquid interface advance 
towards the arc, it increasingly becomes vulnerable to any 
disturbances in the arc form. 

As current increases again in the subsequent pulse, 
growth is arrested and remelting of the growing dendrites 
can also occur. Current pulsing also results in periodic 
variations in the arc forces and hence an additional fluid 
flows, which lowers temperatures in front of the solidifying 
interface. Furthermore, the temperature fluctuations inherent 
in pulsed welding lead to a continual change in the weld pool 
size and shape favoring the growth of new grains. It is also to 
be noted that effective heat input for unit volume of the weld 
pool would be considerably less in pulsed current welds for 
the average weld pool temperatures are expected to be low. It 
is important to note that while dendrite fragmentation has 
frequently been cited as a possible mechanism, and evidence 
for the same has not been hitherto established or 
demonstrated. It has been sometimes suggested that the 
mechanism of dendrite break-up may not be effective in 
welding because of the small size of the fusion welds and the 
fine inter-dendrite spacing in the weld microstructure. Thus 
grain refinement observed in the PCGTAW welds is therefore 
believed to be due to other effects of pulsing on the weld pool 
shape, fluid flow and temperatures. The continual change in 
the weld pool shape is particularly important. As the direction 
of maximum thermal gradient at the solid-liquid interface 
changes continuously, newer grains successively become 
favorably oriented. Thus, the individual grains grow faster in 
small distance allowing for more grains grow, resulting in a 
fine grained structure [15-17, 28]. 

The other declaration of grain refining is that convection 
through the melting pool may be the reason of dendrite 
fragmentation. Dendrite's arms may be separated from the 
main branch during the use of pulsed current. They can act 
as inoculants in fusion zone of weld. These two theories are 
the main reasons of grain refining in fusion zone of PCGTAW. 
Due to the smaller grain size in PCGTAW sample, there are 
more numerous grains; therefore, more grain boundaries exist. 
Due to lower free energy of twin boundaries as compared to 
grain boundaries, the rate of carbide precipitation and 
corrosion rate is low at these boundaries [26] . It is known 
that the discrete and discontinuous distribution of the grain- 
boundary carbides improves the mechanical properties of 
the material, specifically the creep resistance, because these 
particles effectively pin the grain boundaries and decrease 
grain boundary sliding [29] . 

E. Hardness Examination 

The hardness profile versus distance from fusion line is 
shown in Fig. 8. It can be observed that the hardness of weld 
metal decreases as compared to the base metal. During fusion 


AMAE Int. J. on Manufacturing and Material Science, Vol. 02, No. 02, May 2012 

welding and subsequent fast cooling, the precipitates are 
dissolved; therefore, the hardness of weld metal decreases. 
Also, it can be due to grain refinement, precipitation and 
distribution of carbides, size of carbides and residual stress 
[27]. Hardness decreases in the HAZ, since there is no 
allotropic transformation; it is related to grain growth in this 
area. According to Fig. 8, the weld metal hardness of PCGTAW 
sample is greater than that of GTAW samples due to grain 
refining and finer dendrite of PCGTAW mode. 


1. The narrow Inconel 617 welds with smaller HAZ and 
finer grains were produced using pulsed current as compared 
to direct current GTAW. 

20 H<fi 

Fig. 7. Dendrite structures in CCGTAW weld metal 

Fig. 8. Harness test results 

2. In the case of PCGTAW, the mechanical properties 
improved due to grain refinement. The grain refinement can 
be rationalized in terms of lower heat input and pulsed current 
effects. The latter also breaks the arms of dendrites, leading 
to changes of the solidification mode. 

3. The existence of annealing twins and epitaxial growth are 
clear in the base metal microstructures of two samples, the 
boundaries of twins are ordered and show coherent 


The corresponding author gratefully acknowledges the 
financial support of National Iranian Oil Company (NIOC) 
and Pars Oil and Gas Company (POGC). 

©2012 AMAE 5 



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©2012 AMAE